There are well-documented sex differences in tobacco smoking behaviors, a paucity of effective therapeutics, and no gender-sensitive medications. We will evaluate the prototype medication guanfacine, an a2-adrenergic agonist that modulates dopamine (DA) and norepinephrine neurotransmission. The primary aim of Project II is to determine whether the anatomical locus of DA release is sexually dimorphic in tobacco smokers and whether guanfacine differentially modulates DA release by sex. In Aim 1, we will image 20 male and 20 female tobacco smokers with [^^Fjfallypride PET brain imaging to measure changes in synaptic DA in striatal and extra-striatal (frontal cortex, thalamus, amygdala) brain regions. On day 1, all subjects will have a baseline [^(R)F]fallypride PET scan, and on day 2, three hours before the second [ Fjfallypride PET scan, amphetamine (0.5 mg/kg, PO) will be administered. Amphetamine robustly releases extracellular DA, and this paradigm - in which PET radioligand binding after amphetamine is compared to baseline values - will be used as a marker of DA function. We hypothesize that men's response to nicotine is mediated by the mesolimbic DA reward system, whereas, women's response to nicotine may be preferentially mediated by the mesocortical DA system, which is critical to regulate stressors and to self-inhibit. We hypothesize that men will have greater DA release in the striatum, and women will have greater DA release in extra-striatal brain regions, specifically the frontal cortex. In Aim 2, smokers from Aim 1 will be administered guanfacine (3 mg/day, PO, daily, n=20 men, 20 women) for 3 weeks and will be imaged again with the same paradigm. We hypothesize that guanfacine will differentially alter DA release in female and male smokers and that women will have a greater reduction in DA release in the frontal cortex and men will have a greater reduction in the striatum. In Aim 3, we will define relationships between DA release before and after guanfacine treatment and smoking-related reinforcement, stress reactivity, and other smoking behavior related outcomes collected in Project 111 (PI:McKee). The aims of Project II, evaluating sex differences in the neurochemical mechanisms of guanfacine, are highly integrated with Projects I and III which are focused on identifying the underlying neurocircuitry and behavioral mechanisms of guanfacine's effects on stress reactivity and nicotine reinforcement. Synthesis of findings across the projects will identify new biological targets for gender- sensitive therapeutics for smoking cessation. RELEVANCE (See instnjctions): Women compared to men, have much more difficulty quitting smoking and have a poorer response to nicotine replacement therapy, which is the primary first-line treatment for smoking cessation. Despite these sex differences, there is a paucity of research exploring gender-sensitive smoking cessation treatments. This research will contribute to public health by exploring sex differences in potential neurochemical mechanisms involved in tobacco smoking and in the mechanistic effects of guanfacine.